1. Field of the Invention
The present invention generally relates to monolithic microwave integrated circuits (MMICs), and more specifically to a distributed cascode MMIC amplifier arrangement utilizing heterojunction bipolar transistors (HBTs) with balanced electrical parameters and high thermal isolation and heat dissipation.
2. Description of the Related Art
The HBT enables more efficient microwave power amplification than is attainable with field-effect transistors (FETs) such as metal-semiconductor FETs (MESFETs) and high-electron-mobility transistors (HEMTs). The HBT offers high power density, sharp current cutoff and no degradation in voltage breakdown characteristics when operating in the class B/C high efficiency mode. However, the high power density of the HBT results in a smaller device footprint, placing severe demand on the thermal design and circuit layout for MMICs using HBT active elements.
In addition to the thermal problem, the Miller-effect capacitance at the input of a common-emitter HBT amplifier limits the achievable voltage gain and cutoff frequency. A cascode amplifier including a first HBT connected in a common-emitter configuration and a second HBT connected in a common-base configuration is commonly utilized in high power microwave amplification applications to reduce the Miller capacitance and resulting degenerative feedback.
A conventional cascode amplifier 10 is illustrated in FIG. 1 and includes an input NPN type HBT 12 having an emitter connected to ground. A radio frequency (RF) input signal is applied from a source 14 through an input resistor 16 to the base of the HBT 12. The collector of the HBT 12 is connected to the emitter of an output NPN type HBT 20, the base of which is connected to a direct current (DC) bias source 22. The collector of the HBT 20 is connected through an output resistor 24 to a voltage source V+, and also to an output terminal 26. A more detailed description of a conventional cascode amplifier may be found, for example, in a textbook entitled "Integrated Circuit Engineering--Design, Fabrication, and Applications", by A. Glaser et al, Addison-Wesley Publishing Co., 1979, pp. 495-499.
The common-emitter configuration of the HBT 20 substantially reduces the Miller capacitance between the output 26 and an input 36 thereof, reducing the degenerative Miller feedback and increasing the voltage gain and cutoff frequency above those which are attainable with a single common-emitter amplifier. The common-emitter configuration of the HBT 12 provides high current gain, whereas the common-base configuration of the HBT 20 provides high voltage gain without the need for a reactive matching network between the stages. For these reasons, the cascode amplifier is a desirable building block for MMICs.
Each of the cascode amplifier stages may include several smaller HBT cells or units to provide a high power amplifier as illustrated in FIG. 2, in which like elements are designated by the same reference numerals used in FIG. 1. A conventional distributed cascode MMIC amplifier 30 includes a plurality of first stage or input HBTs 32 which collectively correspond to the HBT 12 and a plurality of second stage or output HBTs 34 which collectively correspond to the HBT 20. The emitters, bases and collectors of the HBTs 32 and 34 are respectively interconnected. The collectors of all of the HBTs 32 are connected to the emitters of all of the HBTs 34. This type of distributed cascode MMIC amplifier is described in more detail in "Microwave Journal", Vol. 34, No. 5, May 1991, pp. 189-191.
In the amplifier 30, the input signal from the source 14 is divided among the input stage HBTs 32, amplified and recombined at the collectors thereof. The combined output of the HBTs 32 is divided again among the output stage HBTs 34, amplified and recombined at the output terminal 26. Constraints such as equal in-phase delay of RF signals propagating through the individual HBTs 32 and 34 and equal bias voltage application from the source 22 to the HBTs 34 are imposed on the amplifier circuit design layout. These requirements must be satisfied with a compact circuit configuration having minimum spacing between adjacent HBTs.
On the contrary, high thermal isolation and heat dissipation is achieved by locating the HBTs as far away from each other as possible. These conflicting MMIC circuit design requirements limit the adaptability of the amplifier 30, since the HBTs of the two stages are interconnected at both their inputs and outputs. Reducing the spacing between HBTs to accommodate efficient in-phase signal delay is detrimental to the amplifier thermal properties, and vice-versa.
Another problem with the prior art amplifier 30 is that it is prone to thermal runaway. In a typical design, the HBTs 32 and 34 are biased such that the collector-emitter (collector-ground) voltage drop of the HBTs 32 is relatively low, on the order of 2 volts, whereas the collector-emitter voltage drop of the HBTs 34 is much higher, on the order of 8 volts. Although the collector currents of the HBTs 32 and 34 are equal, the HBTs 34 consume more power than the HBTs 32 due to the higher voltage drop, and thereby generate more heat and operate at a higher temperature.
A fundamental property of bipolar transistors is that as the operating temperature increases, the current flow through the transistor also increases. Above a certain point, the process becomes uncontrollably divergent, and the current flow increases to a level which results in destruction of the transistor. This is the phenomenon of thermal runaway. The configuration of FIG. 2 is especially conducive to thermal runaway since the combined current at the collectors of the HBTs 32 is much greater than the current handling capacity of a single HBT 34.
During normal operation, the current is equally distributed through the HBTs 34 and there is no problem. However, if the HBTs 34 are even slightly mismatched, one HBT 34 will operate at a higher temperature than the other HBTs 34 and draw a larger amount of current. Since the combined current from the HBTs 32 is much more than enough to cause thermal runaway of the hot HBT 34, the possibility that this will occur and that not only one HBT 34, but all of the HBTs 34 will be destroyed in a chain reaction, is greatly enhanced. This is because, as each HBT 34 is destroyed, the remaining HBTs 34 must accommodate not only their own fraction of the total current, but also the current which was previously handled by the destroyed HBTs 34.